Electron Acceptor-Donor Iron Sites in the Iron-Sulfur Cluster of Photosynthetic Electron-Transfer Pathways.

In photosystem I, two electron-transfer pathways via quinones (A _1A and A _1B ) are merged at the iron-sulfur Fe ₄ S ₄ cluster F _X into a single pathway toward the other two Fe ₄ S ₄ clusters F _A and F _B . Using a quantum mechanical/molecular mechanical approach, we identify the redox-active Fe sites in the clusters. In F _A and F _B , the Fe site, which does not belong to the CxxCxxCxxxCP motif, serves as an electron acceptor/donor. F _X has two independent electron acceptor Fe sites for A- and B-branch electron transfers, depending on the Asp-B575 protonation state, which causes the A _1A -to-F _X electron transfer to be uphill and the A _1B -to-F _X electron transfer to be downhill. The two asymmetric electron-transfer pathways from A ₁ to F _X and the separation of the electron acceptor and donor Fe sites are likely associated with the specific role of F _X in merging the two electron transfer pathways into the single pathway.